Driving circuit for printing electromagnet

ABSTRACT

This disclosure relates to a driving circuit for driving a coil of an electromagnet for the actuation of a hammer in a high speed impact printer of data handling systems, which driving circuit includes first switching means upstream of the coil, second switching means downstream of the coil, current sensing means for sensing current in the coil, a closed unidirectional current path including the second switching means, the coil in a diode, first control circuit means for actuating the first switching means in response to an input control signal, second control circuit means for switching on the second switching means in response to the first input control signal and for the entire duration of the input control signal, and a bistable device triggered by the current sensing means to assume one of two electrical states when the sensed current reaches a predetermined value and to provide an output signal corresponding to that one state, the output signal being effective to switch off the first switching means even in the presence of the input control signal.

United States Patent Campari et al.

1 1 Sept. 30, 1975 1 DRIVING CIRCUIT FOR PRINTING ELECTROMAGNET [75]Inventors: Alfredo Campari; Giorgio Vigini,

both of Milan, Italy [73] Assignee: Honeywell Information SystemsItalia, Caluso, Italy [22] Filed: Nov. 19, 1974 [2]] Appl. No.: 525,220

[30] Foreign Application Priority' Data Nov. 28, 1973 Italy 31771/73[52] US. Cl 317/1485 R; 3l7/DIG. 4; 317/31; 317/33 R [51] Int. Cl.'-H0111 47/32; HOIF 7/18 [58] Field of Search 317/123, 148.5 R, 148.5 B,317/149, DIG. 4, 31, 33

[56] References Cited UNITED STATES PATENTS 3,205,412 9/1965 Winston317/D1G. 4 3.206.651 '9/1965 Proulxum 317/DIG. 4 3,235,775 2/1966Winston..... 3l7/DIG. 4 3.549955 12/1970 Paine 3l7/D1G. 4 3,582,7346/1971 Bryden 3l7/D1G. 4

3.859.572 1/1975 Keidl et al. .1 317/D1G. 4

Prinmry limminer-L. T. Hix Atlorney, Agent, or Firm-Fred Jacob [57]ABSTRACT This disclosure relates to a driving circuit for driving a coilof an electromagnet for the actuation of a hammer in a high speed impactprinter of data handling systems, which driving circuit includes firstswitching means upstream of the coil, second switching means downstreamof the coil, current sensing means for sensing current in the coil, aclosed unidirectional current path including the second switching means,the coil in a diode, first control circuit means for actuating the firstswitching means in response to an input con trol signal, second controlcircuit means for switching on the second switching means in response tothe first input control signal and for the entire duration of the inputcontrol signal, and a bistable device triggered by the current sensingmeans to assume one of two electrical states when the sensed currentreaches a predetcrmincd value and to provide an output signalcorresponding to that one state, the output signal being effective toswitch off the first switching means even in the presence of the inputcontrol signal.

6 Claims, 3 Drawing Figures US. Patant Sept. 30,1975 Shet 1 of23,909,681

Sept. 30,1975 Sheet 2 of 2 3,909,681

US. Patent DRIVING CIRCUIT FOR PRINTING ELECTROMAGNET The presentinvention deals with driving circuits for high speed electromagnets,such as those used for hammer actuation in high speed impact printersfor data handling systems.

Such driving circuits must fulfill particular requirements. Thus, theymust provide fast magnetization of the electromagnet, they must haverepeatability with a limited spread of the kinetic energy imparted tothe printing hammer in the various and subsequent energi zations, aswell as during the entire actuation time of the latter.

Among the various-solutions used to achieve such results, there arevoltage controlled energization systems "which use a first highervoltage for energization and a advantages. In particular, the circuitaccording to the invention:

a. provides magnetomotive force sufficient for magnetic core saturationand which assures the appropriate attracting force;

[1. reduces to a minimum extent the power drained from the power supplyand uses to a maximum extent the energy stored in the electromagnetinduc tance;

0. enables the electromagnet to be driven by an unstabilized voltagesupply;

d. reduces the risk of catastrophic damageand may be kept under voltagesupply even in case of failure and has self diagnostic capability.

The driving circuit according to the invention includes in seriesconnection a first and a second current switch respectively upstream anddownstream of the electromagnet winding or coil and a unidirectionalshort circuit path for the coil, in which path the second switch isincluded. The invention further includes a current detector, a bistablecircuit and a control circuit which opens the first switch when thecurrent flowing in the coil reaches a preestablished value.

According to another aspect of the invention the circuit additionallyincludes a second current detector associated with the bistable circuitto detect current flowing through the first switch and to open it whensuch current exceeds a safety limit.

According to still another aspect of the invention, the circuitadditionally includes a voltage detector to provide an indication of thevoltage existing at a coil terminal, the voltage detector and thebistable circuit providing logical information which allows one to checkthe correct circuit operation and, in case of failure, to identifyfaults in the circuit components. '9 Y 9 These and other features oftheinvention will appear more clearly from the following description ofa preferred embodiment and from the attached drawings wherein:

FIG. 1 is a block diagram of a driving circuit according to theinvention. I

'- FIG. 2 shows a wiring schematic of a preferred embodiment of thedriving circuit according to the invention.

. FIG. 3 is a timing diagram of the operation of the circuit accordingto the inventiom Referring now to FIG. 1, the circuit according to theinvention is shown in block diagram form.

The coil 1 of a printing'electromagnet is connected between a voltagesource V and a reference point or ground, through two switches 2 and 3,preferably of the electronic type respectively placed upstream anddownstream'of the coil, and. connected respectively to the windingterminals 4 and 5.

A diode 6 is connected between ground and terminal 4 and is conducting,in relation to the voltage supply, which by example has been chosenpositive, from ground towards terminal 4.

A current detector 7 is interposed between switch 3 and ground and hasan output which controls a bistable device 8. The bistable device 8has-a second control input in the form of an output 10 of a voltagedetecting circuit 9, which provides a signal indicative of the voltageexisting at terminal 5.

The bistable device 8 is set to a first electrical state,

designated set" when the current detected by detector 7 reaches apreestablished value, and is reset to a second electrical state when thevoltage at terminal 5 reaches or exceeds a preestablished value detectedby circuit 9. The two switches 2 and are controlled by control circuits11 and 12, respectively.

When a suitable electrical signal is applied to an input terminal 15 ofthe driving circuit, wires 13 and 16 transfer such signal to the inputsof the control circuits 11,12 which operate to close the switches 2 and3. Switch 3 remains closed for the whole time during which theelectrical control signal is present at input 15. However, switch 2remains closed for a shorter time. This is due to the fact that output14 of the bistable device 8 is connected to a second input of thecontrol circuit 11.

As soon as the current flowing in coil 1 reaches a preestablishd value,which is chosen as being adequate to saturate the magnetic core of thecoil 1, the bistable device 8 issues a signal which is applied tocontrol circuit l1 and commands the opening of switch 2. At this pointthe currentflowing in the coil 1 continues to flow in the closed pathformed by diode 6 and switch 3 and decreases at an exponential ratewhich depends on the time constant of the circuit. When the controlsignal applied to input 15 is removed and switch 3 is opened, theovervoltage due to the opening is detected at terminal 5 by-circuit 9and bistable device 8 is reset. Thereafter, a new operating cycle of theelectromagnet may start.

It is clear from the foregoing that the electromagnet is energized by aninitial voltage which may be chosen so high as tomagnetize and saturatethe circuit in a very short time.

'As soon as magnetizationis achieved, no more energy'is supplied to thecircuit.

If a suitably high voltage supply is used, saturation of the magneticcircuit is achieved before the customary electromagnet keeperaccomplishes a substantial displacementfrom its rest position, so thatthe energy stored in the air gap may be converted nearly in itsentiretyintoakinetic.energyof the keeper. On theother hand, it is knownthat in a magnetic circuit'provided with air gap, the energy'storedinthe gap is'the most relevant fraction of the magnetization energysupplied to the whole circuit. Therefore, the driving circuitaccordingto the invention provides a rational utilization of the energysupplied to the electromagnet. In addition, since the driving-circuit iscontrolled by a quantity (current) relatedto theimagnetization status ofthe coil, it is largely insensitive to changed in the voltage vSwitch 2is now specifically disclosed as a transistor having an emitterconnected to terminal 4 of coil 1. A:

collector of the trnasistor switch 2 is connected to a voltage source+V,, having a rather'high value (for instance 45-V),' through a resistor116 which has a very.

low value (for instance 0.1

Switch 3-is now specifically disclosed as a transistor having itscollector connected to terminal 5 of coil 1 and its emitter connected toground through a resistor 17 which has a very low value (for instance 02G).

Control circuit 1 1, which controls transistor switch 2,comprisestransistors 23,24, pulse transformer 25 and certain resistorsand diodes to be described hereinafter. Transistor 24 has an emitterconnected to ground, while itsbaseis connected to the input terminalthrough a diode 28, whose conducting direction is from the-input to thebase. ln'addition, the base" of transistor 24' is connected toa secondcontrol terminal 31 through a resistor 30.

When, at least one ofthe'control terminals 15 and 31 is at a positivevoltage, transistor 24 is conducting and its collector is'at voltage avery close to ground. Thus, transistor 23, the base of which isconnected-to the collector'of'transis'tor 24, is off and no currentflows in the primary winding of pulse transformer 25. When both theterminals 15 and 13 are at a'voltage close to'ground (or negative)transistor 24 isoff and transistor 23 is cond ucting. Therefore,transformer 25' is fed by avoltage pulse, which is transferred to thesec-" ondary winding and commands the switchingon of transistor switch2. The size of transformer 25 is chosen so as to avoid saturation forthe maximum fo'reseable duration of the control pulses. v

The collector current flowing through resistor 21 causes a'voltag'edrop'across the resistor, so'that the collector volt'age rises. Thus,the base emitter junction of transistor switch 3, is forward biased torender the switch conductive. Transistor switch'3 remains conductive forthe entire duration of the control pulse;

Transistor 23 has itscollector connected to a suitable voltagesource Vthrough the primary winding of the transformer 25 and a current limitingresistor 26. The emitter of transistor 23 is connected to ground. Theterminals of the secondary winding of the transformer 25 are connectedrespectively to the base and Further, the probability of catastrophicfailure is reto the emitter of switching transistor 2. The base ofThevoltage drop produced by the current across the resistor 17 (whichcurrent, byneglecti'ng the negligible base current of transistor switch3,'is also the current flowing in the coil) provides a voltageindication proportional to such current.

In order to assure the demagnetization of the transformer 25, a Zenerdiode 32, connected in opposition to a diode 33,. provides aunidirectional short circuit path.

A. control circuit 12, which controls transistor switch 3, includes atransistor 18, for instance of PNP type,

t and resistors 19, 20, 21, 22. A control terminal .15 is connected tothe base of transistor18 through resistor 22. The emitter of transistorl8is connected to a voltage source V and the collector thereof isconnected both to the base of transistor 103, through resistor 2 and toground, throughresistor 21. i

Normally the input control terminal 15 is kept at a positive voltage sothat transistor 18 isnon-conductive and-the collector of transistor 18is at ground potential Therefore, transistor switch 3 too isnon-conductive. Upon the application of a control pulse to the inputterminal 15, the potential'of the latter approaches ground potential, sothat the base-emitter junction of transistor 18 becomes forward biasedand transistor 18 becomes conductive.

The the use of a pair of switches, enables one to provide the drivingcircuit with diagnostic circuit devices in order to detectand toidentifythe. faults of the same.

duced due to the fact that the probability of a multiple failure of bothswitches is lower than the probability of a simple failure of a switch.

Control terminal 31 is driven by the bistable circuit 8. In a preferredembodiment thebistable circuit 8' includes an amplifier 34, a transistor35 and certain diodes and resistors. The inverting input of theamplifier 34 is connected to the emitter of transistor switch 3 throughresistor 36 and receives a voltage signal proportional'to the currentflowing in coil 1. The noninverting input of the amplifier 34 isconnected on one side to a voltage source +V through resistor 37 and onthe other side to ground, through resistor 38.-ln addition thenon-inverting input of amplifier 34 is connected to the output of theamplifier through resistor 39 and diode 40. Diode 40 has a conductivedirection from the input to the output of the amplifier.

When transistor switch 3 is open, the inverting input of amplifier 34 isat ground potential. Resistors'37 and 38 are chosen of a value so as toapply to the noninverting input a voltage that is slightly positive. Theoutput of the amplifier 34 is therefore at a positive level and diode 40is reverse biased.

When transistor switch 3 is closed and current flows in the coil 1 andin resistor 17, the voltage drop across resistor 17 tends to raise thevoltage applied to the inverting input of the amplifier 34. For asuitable value of the current flowing in the'coil l and corresponding tothe saturation current forthe coil,'the voltage applied to the invertinginput exceeds the voltage applied to the non-inverting input and theoutput-voltage becomes negative. Diode 40v becomes conductive and lowersthe voltage applied to the non-inverting input, causing a positivefeed-back which saturates the amplifier 34 and maintains it in thatstate even when the current in resistor 17 goes to zero and thevoltage'applied tofth'e inverting input goes to ground.

The output of amplifier 34 is connected through resistor 41 to the baseof transistor 35, whose collector is connected to the voltage sourcethrough resistor 43 and whose emitter is connected to, ground. A diode44 between the base of transistor 35 and ground, the

direction of conduction of which from ground to.base,'

prevents the application .of too high a reverse bias to the base oftransistor 35. The collector of transistor is further connected to theinput terminal 31. When the, voltage at the output of amplifier34 ispositive, transisrespective of the voltage applied to terminal 15,transistor 24 is kept in its conductive state and transistor 23 andtransistor switch 2 are switched off. Therefore, as

soon as the current flowing in the electromagnet, of

which coil 1 is a part, reaches a predetermined value suitable tosaturate the magnetic core, the abovedescribed circuits intervene tocause the switching off of transistor switch2 to be switched'off and toremain non-conductive in its stable condition.

However, the current flowing in coil 1 is not switched off and theself-induced electromotive force in the circuit sustains the currentwhich may flow through diode 6. Only at the termination of thecontrol-pulse applied to terminal '15, when transistor switch 3 isopened, is the residual current flowing in the coil 1 switched off. Theself, induced voltage caused by this switching action is used to resetthe bistable circuit 8. To this purpose, such voltage is appliedthrougha voltage divider formed by series, connected resistors 46,47 (which areconnected on one side of terminal 5 and on the other side to ground) andthrough diode 45 to the noninverting input of amplifier 34. Such voltageis sufficiently high to bring the potential of the non-inverting inputto a potential higher than the one applied to the inverting input sothat the output of the amplifier 34 becomes positive. In order to limitthe self-induced voltage which is generated by transistor switch 3switching off, terminal 5 is connected to the voltage source V throughdiode 42 so that the self-induced voltage cannot exceed the value ofvoltage source V The remainder of the driving circuit consists ofprotective devices. Maximum current protection is achieved by means ofresistor 1 l6, transistor 48 and resistor 49. Transistor 48 (of PNPtype) has its emitter connected to the voltage source V and itscollector connected to the inverting input of amplifier 34, throughresistor 49. Its base is connected to the collector of transistor switch2.

If, for some reason the current flowing in resistor 116 exceeds apredetermined value, transistor 48, normally open, starts conducting andapplies to the inverting input of the amplifier 34 a positive voltagewhich causes the intervention of bistable circuit 8 and the switchingoff of transistor switch 2.

Some examples of causes of failure to bring about the foregoing actionare as follows:

a. short circuit of diode 6.

b. short circuit of coil 1.

c. transistor switch 2 emitter grounded.

A further device which provides indirect protection consists of avoltage detector connected to terminal 4 of the coil 1. Such a circuitmay consist of a voltage dividerincluding resistors 50, 51 and athreshold circuit 52 having an input connectedto such voltage divider soasto provide an output signal of a first level when the input voltage islower than, a threshold voltage, and an output signal of a second levelwhen the input voltage is higher than the threshold voltage. Thethreshold circuit 52, as well'as amplifier 34 are conventional circuitswhich are commerciallyavailable as integrated devices and therefore theyneed not be described in detail.

An output terminal E of the threshold device 52 provides'asignal which,jointly with the signal present at the collector of transistor 35(terminal D), provides a heretofore unavailable degree of diagnosticpower for detecting failures and for fast tion circuit. 7

FIG. 3 is a timing diagram showing the operation of the driving circuitsand enables one to evidence the utility of signals present at terminalsD, E. In the diagram, the waveform designated by the reference numeral15) represents the control voltage applied to input terminal 15. Thewaveforms designated D) and E)'represent the signals at' terminals D andE respectively, while I) represents the current flowing in coil 1.

At the beginning, theinput control signal is at a positive value, nocurrent is flowing in the coil 1 and terminals and D are at' zerovoltage. At instant t the input control signal is lowered to zero level.At instant t,, with a delay which depends on the intervention time ofthe control circuits, transistor switches 2 and 3 start conducting andcurrent starts flowing in coil 1. There fore, at instant t the signal atterminal E rises to a positive level. When the current I) reaches apredetermined value (instantt the bistable circuit 8 is set and thesignal at terminal-D'rises to a positive level. Therefore, transistorswitch 2 is cntrolled to switch off, such action occurring with somedelay at instant 1 At instant t the voltage at terminal 4 decreases andthe signal at terminal E goes to zero level.

With a predetermined delay from T at 1 the input control signal israisedagain to a positive level so as to command the-opening of transistorswitch 3. This occurs with some delay starting from instant t in whichthe self-induced voltage caused by the circuit opening resets thebistable"v circuit 8, lowering to zero level the signal at terminal D.

In normal operation, at instant t the voltage at terminals ED is zero,while at instant t the voltage at D is positive and the voltage at E iszero. Voltage levels different from the ones indicated above anddetected at instants t t, are indicative of failures.

Among the serious failures which may occur are the following:

a. Short circuit of transistor switch 2. It causes the terminal 4 toremain in tension, so that at instant! signal E remains at positivelevel.

b. Open steady state of transistor switch 2. It prevents theenergization of the coil 1, so that the bistable circuit 8 is nottriggered at instant t At instant 1 signal D is still at zero level.

0. Short circuit of diode 6, 0r transistor switch 2 emitter grounded. Itcauses, as already seen, the intervention of the protection circuit andthe set of bistable circuit 8. On the other hand there is no currentflowing in the coil 1 so that bistable circuit 8 is not reset. Thereforeat instant t of a subsequent activation of the protec- 7 input controlpulse'the voltage level at terminal D will be. positive instead of zero.

d. Transistor switch 3 steadily in off state. No current can flow in thecoil 1: the bistable circuit 8 is not set and at instant t terminal D isstill at electrical "levelzero. v

From 'the foregoing description, it will be clear that the circuitdescribed, which forms the subject matter of applicants invention,provides a number of advantages, as discussed above. Specifically, theinvention provides greater efficiency, reliability-and safety thanheretofore available in comparable circuits, as well as in improvedability to diagnose failures.

The driving circuit described constitutes a preferred form of embodimentof the invention. Numerous changes, modifications and substitutions willnow occur to those skilled in the art, all of which fall within thespirit and scope of the invention, as, defined by the ating said firstswitching means in response to an input,

control signal; second control circuit means connected to said secondswitching means for switching on said second switching means in responseto said input control signal and for the entire duration of said inputcontrol signal; a bistable deviceconnected to and triggered by saidcurrent sensing means to assume one of two electrical states when thesensed current reaches a preestablished value and to provide an outputsignal corresponding to said one state, said output signal beingconnected to said first control circuit means and effective to switchoff said first switching means even in the presence of said-inputcontrol signal.

2. Driv.ing circuit as claimed in claim 1 further comprisingovervoltagedetecting means to detect an overvoltage due to the switching off ofsaid unidirectional current path and to provide an overvoltage signal,said overvoltage signal being applied to said bistable device, saidbistable device being reset in that one of the electrical states otherthan said one by said overvoltage signal.

3. Driving circuit as claimed in claim 1 wherein there is a voltagedetector connected to said coil between said coil and said firstswitching means.

4. Driving circuit as claimed in claim 3 wherein there is voltage signalmeans connected to and operated in response to the condition of saidbistable device for diagnosing element failure.

5. Driving circuit as claimed in claim 1 wherein there is voltage signalmeans connected to and operated in response to the condition of saidbistable device for diagnosing element failure.

6. Driving circuit as claimed in claim 1 wherein said first controlcircuit means includes a transformer having a primary winding, anddemagnetization means for said primary winding including means definingan unidirectional short circuit'path.

1. Driving circuit for driving an electromagnet coil with a controlledcurrent comprising: first switching means upstream of said coil andconnected thereto; second switching means downstream of said coil andconnected thereto, current sensing means connected to said coil forsensing the current in said coil; a closed unidirectional current pathincluding said second switching means, said coil and a diode; firstcontrol circuit means connecteD to said first switching means foractuating said first switching means in response to an input controlsignal; second control circuit means connected to said second switchingmeans for switching on said second switching means in response to saidinput control signal and for the entire duration of said input controlsignal; a bistable device connected to and triggered by said currentsensing means to assume one of two electrical states when the sensedcurrent reaches a preestablished value and to provide an output signalcorresponding to said one state, said output signal being connected tosaid first control circuit means and effective to switch off said firstswitching means even in the presence of said input control signal. 2.Driving circuit as claimed in claim 1 further comprising overvoltagedetecting means to detect an overvoltage due to the switching off ofsaid unidirectional current path and to provide an overvoltage signal,said overvoltage signal being applied to said bistable device, saidbistable device being reset in that one of the electrical states otherthan said one by said overvoltage signal.
 3. Driving circuit as claimedin claim 1 wherein there is a voltage detector connected to said coilbetween said coil and said first switching means.
 4. Driving circuit asclaimed in claim 3 wherein there is voltage signal means connected toand operated in response to the condition of said bistable device fordiagnosing element failure.
 5. Driving circuit as claimed in claim 1wherein there is voltage signal means connected to and operated inresponse to the condition of said bistable device for diagnosing elementfailure.
 6. Driving circuit as claimed in claim 1 wherein said firstcontrol circuit means includes a transformer having a primary winding,and demagnetization means for said primary winding including meansdefining an unidirectional short circuit path.